This is about the standalone FMD app, not the built-in Find My Device function.
Install FMD from F-Droid
Mostly you can just follow the documentation about setting up.
However, if you want to use the FMD server to control and find your device, you need Push notification. The FMD documentation says that you need a distributor app and suggests that you should install sunup.
You don’t need to do this, because the ntfy distributor app is already installed. Just click “Register” or “Register again” in the FMD Server section of the app settings.
Aside: The built in ntfy lacks any GUI. To control it, install Activity Manager from F-Droid, open it and find,click ntfy in the list of applications, click MainActivity io.heckel…